Due to the high number of implanted hip and knee prostheses at an increasing age of the operated patients, the frequency of periprosthetic or interprosthetic fractures is increasing.
Periprosthetic fractures are fractures in the area of a prothesis and are often caused by a fall. Also, a low bone quality due to osteoporotic or osteolytic modifications may lead to a periprosthetic fracture.
Interprosthetic fractures occur between two implants or prosthesis inserted in the same bone. Interprosthetic fractures are most often found in the femur between a hip and a knee prothesis.
Depending on the kind and profile of a fracture, its location with respect to the prosthesis and the dependence on the seating stability of the prothesis, various therapy forms are indicated.
In particular, in order to choose the type of fracture care, it is decisive, whether the seating of the prothesis has come loose. If the prothesis is still solidly anchored to the bone, in most cases a revision of the prosthesis may be omitted.
The standard method for the treatment of periprosthetic or interprosthetic fractures, which are categorized as type B1 according to the Vancouver classification, is represented by the angularly stable plate osteosynthesis. However, the treatment by plate osteosynthesis is responsible for the weakening of the prosthetically treated bone due to formation of predetermined breaking points, in the area of which successive fractures may occur. Depending on the specific requirements the use of a megaprosthesis as a complete femur insert may then be inevitable.
If the bone structure does not allow an osteosynthesis, the intramedullary stabilization represents an alternative operational technique, in which the defect is bridged by means of an individually manufactured interposition-nail.
Interposition nails offer a high stability and are manufactured in different sizes.
In case of fractures of type Vancouver B1, in which despite the fracture the hip or knee prosthesis is still solidly anchored to the bone, a further care method may be used, in which the implanted prosthesis may remain within the body. In this operational technique, for example, in order to bridge the bone defect in the femur, the shaft of an implanted hip prosthesis is connected to a femur nail, which is inserted distally into the medullar channel, by means of a sleeve-like intermediate module, whereby the shaft may be mechanically extended. This methodology has various advantages. On one hand, the duration and depth of the surgical intervention may be considerably reduced, and on the other hand, an improved healing and higher stability of the prosthetically treated bone may be considered, with a simultaneous reduction of the complication rate.
However, only a few modular endoprosthetic systems are available, in which the implant or prosthesis components are fitted to each other in a way that an already implanted prosthesis may be endoprosthetically expanded in a stable way in the course of a subsequent operation.
It is often the case that both the origin of a prosthesis shaft and its geometry and consistency may be univocally determined only when it has been operatively exposed. The surgeon then has to make a rapid assessment regarding whether a prepared endoprosthetic system is compatible with the pre-existing shaft and whether the subsequently implanted endoprosthetic system may be connected to the shaft end in a rotationally stable way.
The market for medical products offers a large variety of hip and knee prostheses, the configuration and size of which are clearly differentiated from each other. The fundamental differences are in particular in the length and in the cross-sectional profile of the prosthetic shafts and in their respective superficial texture. Some prostheses are provided, among other things, with corrugations or longitudinally extending groove profiles, which hinder an intramedullary expansion to an endoprosthesis due to an insufficient force fit between the prosthetic parts.
Endoprosthetic connection systems are known in the art which allow an endoprosthetic extension of exposed shaft ends after a periprosthetic fracture.
DE 10 2008 062 226 A1 discloses an extension of a proximal femur nail. The extension in the form of a distal femur nail, which may be inserted into the medullar space of the femur in a retrograde way, has a receiving opening at a proximal end, which opening may be slid over the distal end of the proximal femur nail. The securing occurs by means of a locking screw, which is additionally connected to the femur.
U.S. Pat. No. 8,668,692 B1 described a periprosthetic endoprosthetic system for an already implanted prosthesis shaft. This endoprosthetic system comprises a connection part, which has a longitudinal tripartite channel. The proximal area of this channel is provided for receiving the inner shaft end and adapted to the conical shaft shape. The connection between the inner shaft end and the conically shaped channel region occurs through force fit, with or without an additional cementing of the shaft end. In the distal end region of the channel a further implant may be screwed in for an endoprosthetic extension.
DE 39 09 182 C1 is also cited as a correlated state of the art.